Spinel is an important class of oxides consisting of two types of metal ions with the oxide ions arranged in ccp layers. The normal spinel has one-eighth of the tetrahedral holes occupied by one type of metal ion and one-half of the octahedral holes occupied by another type of metal ion. Such a spinel is formed by Mg2+ , Al3+ and O2−. The neutrality of the crystal is being maintained.
If oxide ion is replaced by X−8/3, the number of anionic vacancies per unit cell is :